Add BuildTargets.md

[lukewagner]

This PR adds BuildTargets.md to define this new concept of "build targets" as presented in both CG-06 and WASI-06-12, which itself was a revision of the earlier "wasit2" idea that came up in WASI/#595.

Currently, BuildTargets.md only defines one build target, wasm32, but with the intention of adding more in the future (e.g., wasm64 and wasmgc). See BuildTargets.md in this PR for more details.

The goal, mentioned at the end of BuildTargets.md, is that wasi-sdk and various other currently-component-producing toolchains would be able to emit core modules that matched wasm32 as defined here (with "componentification" being merely a final linker option of whether to run wasm-ld or wasm-component-ld). I plan to leave this open until we can get some implementation feedback to test this out.